Atherosclerotic vascular disease is the leading cause of death and morbidity in industrialized nations. Understanding the biochemical mechanisms leading to atherosclerosis may lead to new therapies and preventative measures. Levels of myeloperoxidase (MPO) and MPO-derived products have been shown to be elevated in atherosclerotic tissue and potentially mediate pro-atherosclerotic mechanisms. Upon activation, monocytes and macrophages release MPO and produce its substrate, hydrogen peroxide, resulting in MPO-catalyzed production of the reactive chlorinating species (RCS), hypochlorous acid. We have discovered that RCS target the vinyl ether bond of the plasmalogen phospholipids resulting in 1-chlorofatty aldehyde (1-ClFALD) production and have shown that 1-ClFALD levels are increased 1400-fold in human atherosclerotic tissue. We found that 1-ClFALD is metabolized to 1-chlorofatty acid and 1-chlorofatty alcohol, which along with 1-ClFALD are primary members of a novel chlorinated lipidome. Preliminary studies show that 1-chlorofatty acid: 1) is elevated in activated human monocytes and in human atherosclerotic tissue; 2) inhibits endothelial nitric oxide synthase; 3) elicits ceramide accumulation in macrophages and smooth muscle cells; 4) causes macrophage apoptosis; 5) stimulates the production of pro-inflammatory cytokines by macrophages and 6) is present in human plasma. In this proposal we will test the hypothesis that 1-ClFALD metabolites are mediators of atherosclerosis. This hypothesis will be tested by three specific aims. Specific Aim 1 will test the hypothesis that 1-ClFALD metabolites are candidate mediators and biomarkers of atherosclerosis. Mass spectrometry- based screens will be employed to identify 1-ClFALD metabolites in human monocytes and macrophages, as well as human atherosclerotic tissue. A case-control study will be performed to identify metabolites of 1- ClFALD as biomarkers of human coronary artery disease. 1-ClFALD metabolites will also be identified in atherosclerotic tissue and plasma from LDL receptor-/- mice on an atherogenic diet that overexpress human MPO in their macrophages. Specific Aim 2 will test the hypothesis that 1-ClFALD metabolites mediate pro-inflammatory and proatherogenic mechanisms. Mechanisms responsible for chlorinated lipid mediated eNOS dysfunction will be examined. The role of 1-ClFALD metabolites as mediators for the release of pro-inflamma- tory cytokines will be examined. A sub-hypothesis of Aim 2 is that chlorinated lipid metabolites increase de novo ceramide biosynthesis and subsequent apoptosis. Specific Aim 3 will test the hypothesis that chlorinated lipids cause atherosclerosis. Atherosclerosis will be examined in apo E-/- and LDL receptor-/- mice that are fed chow and pro-atherogenic diets that are supplemented with 2-chloropalmitic acid. Plasma and urine levels of chlorinated lipid metabolites, as well as the level of atherosclerosis, will be compared to those levels in both hMPO and MPO-/- mice on pro-atherogenic diets in studies from Aim 1. Taken together, the proposed studies will use a multi-disciplinary approach to identify chlorinated lipid species as novel mediators of atherosclerosis.